There are a number of benefits to being able to accurately estimate the velocity of a sea-faring vessel. One is that an accurate estimation of velocity results in improved estimates of ship's position (when those estimates are velocity-based) for submerged vehicles (e.g., submarines, AUVs, UUVs, etc.). This is particularly important for submerged vehicles for which Global Positioning System (GPS) fixes are not available or otherwise kept to a minimum to maintain ship's covertness. Another benefit is that it improves the accuracy of certain on-board missile-delivery systems that employ a technique to obtain a velocity fix, which is then provided as initialization data to the missile before launch (i.e., it reduces missile Circular Error Probable (CEP)).
It is known to apply signal correlation to SONAR technology to measure velocity. See, for example, U.S. Pat. No. 4,244,026 to Dickey and U.S. Pat. No. 5,315,562 to Bradley et al. These systems typically include a sonar source and multiple receivers (i.e., hydrophones), which have a known separation. The SONAR source directs sonic pulses towards the ocean floor, and the receivers detect echoes of those pulses. The velocity of the vessel is then calculated based upon the distance traveled by the vessel between the transmission and reception of a first pulse and a second pulse.
As discussed further below, correlation SONARS rely on selecting a best or maximum “correlation” either between hydrophones or pulses, for the determination of velocity. Maximum correlation occurs when the ray path of an initial SONAR transmission (from the transmitter to the ocean floor, etc., and back to a receiver) of a first detected pulse is equal to the ray path of a second SONAR transmission.
Correlation SONAR systems can be water or ground referenced, and Spatial or Temporal based. A water-referenced correlation SONAR uses echoes reflected from the water beneath a vessel, whereas a ground-referenced correlation SONAR uses echoes reflected from the ocean bottom. A correlation sonar can also be both ground and water based in the sense of having both ground- and water-referenced modes of operation.
Spatial correlation SONAR calculates the velocity of a vessel by transmitting two or more pulses towards the ocean bottom, detecting echoes of the pulses on a planar two-dimensional array of hydrophones, determining which two hydrophones in the array correlate the best, and dividing the distance between those hydrophones by twice the time differential between the pulses. Peak correlation might take place between hydrophones, in which case an interpolation scheme is used. A Temporal correlation SONAR determines velocity by transmitting multiple pulses toward the ocean bottom and detecting echoes of the pulses at a hydrophone array. For a given pair of hydrophones, the system determines which two pulses correlate the best, and calculates velocity by dividing the fixed distance between the hydrophones by twice the time differential between the two correlated pulses.
Velocity estimates from correlation SONAR are subject to a variety of different random errors and bias errors. To the extent that these types of errors can be reduced, the accuracy of the velocity estimates will improve. Correlation SONARS also have integrity issues in which serious performance degradation can occur in the event that there is an undetected failure in a hydrophone or hydrophone channel and the SONAR uses the faulty channel data for its velocity solution. The phrase “hydrophone channel” means the hydrophone itself, as well as the connectors and cabling to channel electronics, the electronics, and associated data-processing components.